Electric Charges

The basic sub atomic particles are the Proton,
Electron, and Neutron. These particles are the constituents of the atoms
of all the chemical elements. A simple pictorial representation of an
atom is presented in the drawing at the left. These three particles
represent the kinds of reactions that matter can have with respect to
electric interactions. In fact, two electrons repel each other,
likewise, two
protons repel each other, while neutrons do not interact electrically
among themselves nor with electrons and protons. On the other side, between electrons
and protons, there is electric attraction. The property that particles
possess that accounts for their electric interactions is called
charge. Based on the previous interaction among charges, it is
concluded that there are two types of charges;
positive (+) assigned to the
proton, and
negative (-) for the
electron. The neutrons are
uncharged or neutral.

Experimentally, it has been established that
the magnitude of the charge of an electron is
the same as the magnitude of the charge of a proton. Therefore, the
atom on the drawing above is a neutral atom.

The first experiments in electricity were in what is called
electrostatic. Electrostatic studies the interaction among charges after the
charges can be considered at rest, i.e., without motion. Usually, when a charge
is placed in the vicinity of other charges, a re-distribution of charge may
occur. Electrostatic studies the interaction among the charges after the
re-distribution of charges has occurred.

One such experiment is to rub a rubber rod with a
piece of fur (cat or rabbit). During the rubbing, charges are exchanged
between the rod and the fur. In fact, electrons are taken from the fur
and transferred to the rubber rod leaving the fur with an excess of
positive charge and the rubber rod with an excess of negative charge. Similarly, when rubbing a glass rod with a
silk rug results in electrons being removed from the glass rod and transferred
to the silk rug. In this case, the glass rod ends with an excess of positive
charge while the silk rug ends up with an excess of negative charge.

With the help of the two rods, rubber and glass, different
electrostatic experiments can be set. Repulsion and attraction of charges can be
visualized with the following drawings:

The drawings of the sequence represent two small metallic
spheres (conductors) hanging from
insulating strings, such as fishing line (insulator).

The first drawing represents the spheres without charge.
There is not electric interaction between the spheres and they just hang
vertically due to their weight.

The second drawing represents the same spheres after both of
them have been touched by a negatively charged rubber rod. Because of the
electric repulsion between the charges, the two spheres move away from each
other. The third drawing also represents electric repulsion but, in this case,
between positive charges. Here, the spheres have been charged by touching them
with a positively charged glass rod.

The last drawing shows electric attraction between the
spheres. In this case, one of the spheres (left) has been touched with the
positively charged glass rod while the other (right) has been touched with the
negatively charged rubber rod.

Unit of Charge

The first unit of charge was defined as a hundred rubs on
cat fur with an amber rod (Classical Electrodynamics 1st Ed., John D. Jackson, section
1.2). Later, the unit of charge was called the Coulomb, C. Modern definitions of
the Coulomb are based on the current (see
electric current).

Using this unit, the elementary charge is
. This charge corresponds to the magnitude of the charge of an electron and also to
the charge of a proton. As indicated above, the magnitude of the charge of an electron is the
same as the magnitude of the charge of a proton; consequently, atoms with the
same number of protons as electrons are
neutral. Atoms with different number of electrons than protons are
charged and called ionized.

Conservation of the Charge

Experimentally, it is observed that in all charging and
discharging of objects, there is only exchange of charges between objects. Thus,
the combination of charge of the involved objects (total charge) is unchanged. Based
on these experimental results, it is said that the electric charge is conserved and,
in nature, the electric
charge can not be created nor destroyed.

Quantization of the Charge

Also experimentally, it is observed that the smallest amount
of free charge that can be found is the elementary charge,
.
Since no fraction of the elementary charge can be found, the electric charge is
quantized. Thus the charge of an object can be